1. Field of Invention
The present invention relates to a multi-phase switching regulator and a droop circuit therefor; particularly, it relates to such multi-phase switching regulator and droop circuit which can sense a total current generated by multiple switch sets and adjust an output voltage accordingly.
2. Description of Related Art
FIG. 1 shows a schematic diagram of a multi-phase switching regulator disclosed by U.S. Pat. No. 6,683,441. As shown in FIG. 1, a summing circuit 25 includes two resistors having the same resistance Rp. Each of the two resistors has one end connected to a corresponding phase node, and the other end connected to a summing node 26 in common. An amplifier circuit includes an operational amplifier A1, which has an inverting input connected to the summing node 26, and a non-inverting input connected to an output voltage Vout. A resistor Rcs (having a resistance Rcs), which is connected in parallel with a capacitor Ccs (having a capacitance Ccs), sets the gain of the operational amplifier A1. When so arranged, the output voltage Vcs produced by the operational amplifier A1 is given by:
      V    cs    =            V      out        -                                        R            cs                    ⁢                                    R              1                        ⁡                          (                              1                +                                  s                  ⁢                                      L                                          R                      1                                                                                  )                                                            R            p                    ⁡                      (                          1              +                                                sC                  cs                                ⁢                                  R                  cs                                                      )                              ⁢              I        out            wherein L is the inductance of the inductor; R1 is the parasitic resistance of the inductor; s is a variable of Laplace Transform, and lout is the total output current. If the time constant Ccs*Rcs is made substantially equal to the time constant L/R1 of the inductor, and the output voltage Vout term is subtracted from the output Vcs of the operational amplifier A1 by a summing circuit 30, which receives Vcs at one input, Vout at another input, a droop voltage Vdroop is generated which is given by:
      V    droop    =                    V        cs            -              V        out              =                  -                                            R              cs                        ⁢                          R              1                                            R            p                              ⁢              I        out            
Thus, Vdroop is directly proportional to the total output current lout. The droop voltage Vdroop may be used to provide various functions. For one example, over current protection (OCP) can be achieved by detecting the total current. For another example, in some applications it is required to control the relationship between the output current and the output voltage. In such case, the output voltage Vout can be adjusted according to the droop voltage Vdroop, to achieve the so-called droop control.
The aforementioned prior art requires a large capacitance Ccs and a large resistance Rcs, so the capacitor and resistor can not be integrated in an integrated circuit (IC) chip, and the IC chip needs to be provided with pins for connecting with the capacitor and the resistor. Certainly, this will increase the cost.
FIG. 2 shows a schematic diagram of a droop circuit disclosed by U.S. Pat. No. 7,064,528. As shown in FIG. 2, multiple phase nodes PH1-PHN are coupled to corresponding resistors R1-RN respectively, and the other ends of the resistors are coupled to a non-inverting input of an operational amplifier A2 in common. The output voltage Vout is coupled to an inverting input of the operational amplifier A2 via a resistor RA. A capacitor CA is coupled between the non-inverting input of the operational amplifier A2 and the output voltage Vout. A feedback resistor RB is coupled between the inverting input of the operational amplifier A2 and an output end of the operational amplifier A2. A feedback resistor RC is coupled between the non-inverting input of the operational amplifier A2 and the output voltage Vout. A capacitor CB is coupled between the output voltage Vout and the output end of the operational amplifier A2. The droop voltage Vdroop is the voltage between the output end of the operational amplifier A2 and the output voltage Vout.
Similarly, the droop voltage Vdroop may be set proportional to a total output current (not shown) by proper settings of various parameters of the devices of the droop circuit, such that the droop voltage Vdroop may provide functions such as OCP, droop control, or serve for other purposes.
FIG. 3 shows a schematic diagram of a droop circuit disclosed by US Publication No. 2009/0051334. As shown in FIG. 3, multiple phase nodes PH1-PHN are coupled to corresponding resistors RPH1-RPHN respectively, and the other ends of the resistors are coupled to a non-inverting input of an amplifier 520. The output voltage Vout is coupled to an inverting input of the amplifier 520 via a resistor RCS. A capacitor C1 is coupled between the non-inverting input of the amplifier 520 and the output voltage Vout. An output end of the amplifier 520 generates a droop current as below:
      I    droop    =                              I          out                ×        DCR            N        RCS  wherein N is a number of the phase nodes; lout is the total output current; DCR is a parasitic resistance of each inductor L1-LN. The droop current Idroop is proportional to the total output current lout, so it can be used for functions such as OCP and droop control.
In the prior art circuits shown in FIG. 2 and FIG. 3, a current path is formed between the amplifier and the output node. If the current path is required to detect a negative current, an offset voltage needs to be provided in the current detection circuit. This offset voltage will deliver a current to the output voltage under discontinuous conduct mode (DCM) or in low load condition, thus charging the output voltage to an undesired high voltage level. In other words, these two prior art circuits have the problem of output voltage instability, in particular in the aforementioned case.
In view of the above, to overcome the drawbacks in the prior art, the present invention proposes a multi-phase switching regulator and a droop circuit for the multi-phase switching regulator, which increase the flexibility of the circuit design and improves the stability of the output voltage.